Such a propeller combination and a boat propeller drive unit of so-called inboard-outboard type having such a propeller combination is shown and described in SE 433599. Drive units of this type are common in planing motorboats of a length up to about 40 feet and for a speed range of 25-50 knots, where they provide, with few exceptions depending on boat type, higher efficiency and more rapid acceleration than corresponding single propeller drive units. Other advantages over known single propeller drive units are also achieved.
The individual propellers in this known propeller combination are dimensioned to provide essentially equal pressure force and in order to achieve approximately the same security against cavitation, the propellers are dimensioned with approximately the same total blade area. Despite this the blades of the after (aft) propeller, after operating for an extended period, have at times been subjected to degradation, particularly on the pressure side, to a greater extent than the forward propellers.